The present invention relates generally to radio communication devices, and specifically to devices and methods for accurate frequency control of radio transceivers.
Radio transceivers used in mobile communication require stable clock sources and frequency synthesizers for use in modulation and demodulation of RF signals. The frequency synthesizers must be capable of tuning and locking onto a base station frequency with high accuracy. In a typical cellular communications system, for example, a radio transceiver must typically be capable of initial tuning, based on dead reckoning to an accuracy of 2 ppm, and then once a base station signal is detected, locking onto the signal frequency to an accuracy of 0.2 ppm. In the 800 MHz cellular band, these requirements translate into 1.6 kHz initial tuning accuracy and 160 Hz locking accuracy. In the 1.5 GHz band, the figures are roughly double.
Crystal oscillators, which are commonly used for frequency control in radio transceivers and other applications, are not in themselves sufficiently stable to provide the levels of accuracy noted above. The inherent frequency of the crystal oscillator is known to vary with temperature and also to change gradually as the crystal ages. A typical crystal oscillator, for example, a PXO-type oscillator, produced by Telequarz of Neckarbischofsheim, Germany, has a frequency accuracy of xc2x110 ppm. The long-term drift of the frequency of the crystal with age is about 2 ppm per year.
Because of these shortcomings of ordinary crystal oscillators, transceivers known in the art generally use either a temperature-compensated crystal oscillator (TCXO) or a voltage-controlled TCXO (VCTCXO) as a precise frequency reference source. Temperature-compensated oscillators are described, for example, in U.S. Pat. Nos. 3,938,316, 4,015,208, 4,454,483, 5,375,146 and 5,604,468, which are incorporated herein by reference. The TCXO or VCTCXO receives an indication of the ambient temperature and corrects the oscillator output frequency accordingly, so as to compensate for the known characteristic temperature variation of the crystal frequency.
TCXOs and VCTCXOs are larger and heavier than simple crystal oscillators and include elements that are not easily incorporated in monolithic transceiver devices. They also depend on the use of a crystal which has a high motional capacitance to allow for externally-controlled tuning of the oscillator, and is therefore larger than standard crystals used in simple crystal oscillators. Therefore, TCXOs and VCTCXOs add undesirably to the cost and weight of RF communication devices, such as cellular telephones, in which they are used.
It is an object of some aspects of the present invention to provide methods and devices enabling accurate tuning of a radio transceiver using a low-cost frequency reference source.
It is a further object of some aspects of the present invention to provide an accurate radio frequency source that can readily be incorporated in a monolithic transceiver device.
It is yet another object of some aspects of the present invention to provide an accurate radio frequency source that does not require a TCXO or a VCTCXO.
In preferred embodiments of the present invention, a radio transceiver, which communicates with a base station on a base station carrier frequency, comprises one or more frequency synthesizers, which receive clock signals from a reference oscillator. Preferably, the reference oscillator comprises a simple crystal oscillator without temperature compensation or means for external frequency control. A frequency error of the clock signals relative to a clock frequency required by the radio transceiver is estimated based on the error it causes in the frequency of the transceiver. The estimated error is parsed into a coarse correction estimate and a fine correction estimate. The coarse correction is applied to step one or more of the frequency synthesizers, so that the synthesizers generate frequency signals within a predetermined range of a target frequency. The fine correction is applied by generating a corresponding frequency correction in a correction circuit of the transceiver, preferably in baseband circuitry of the transceiver. By parsing the frequency error and thus applying frequency corrections cooperatively in the synthesizers and the baseband circuitry, the present invention enables the transceiver to tune and lock onto the carrier frequency with high accuracy, while maintaining a fast time response and low noise level, without the need for a temperature-compensated or voltage-controlled oscillator.
In some preferred embodiments of the present invention, additional information is input to the transceiver regarding physical conditions of the crystal oscillator. This information preferably includes the temperature and age of the crystal, as well as predicted or recorded data regarding the response of the crystal oscillator to the physical conditions. The information may also include the oscillator supply voltage and load, as well as any other parameters affecting the performance of the oscillator. The information is used in dead reckoning of the frequency error, which is applied in initial tuning of the transceiver. Thereafter, a closed-loop measurement of the error is preferably made, and the measured error is parsed into a coarse frequency error and a fine residual error and is used in fine-tuning the synthesizers and baseband circuitry.
In some preferred embodiments of the present invention, the frequency correction of the baseband signals is performed by shifting the phase of the signals, most preferably by subtracting the residual frequency error after demodulation of intermediate frequency (IF) signals. Alternatively or additionally, the residual frequency error is corrected by multiplication of the modulated IF signals by a phasor, either in digital or analog form, using a complex multiplier. The phasor applies to the signals a phase shift that varies continuously with time, in a manner calculated to generate the desired frequency correction.
Further alternatively or additionally, the residual frequency error is accounted for during generation of the signals (in transmission) and/or in determination of the values of the signals (in reception), preferably at the stage of modulation and/or demodulation of the signals. For example, in a system utilizing FSK modulation, in which the demodulation is performed by a frequency discriminator, the decision threshold level is adjusted responsive to the residual frequency error. During transmission, the transceiver generates modulating tones at a frequency that differs from the designed frequency by an amount necessary to account for the residual error.
There is therefore provided in accordance with a preferred embodiment of the present invention, a radio communication device, which transmits or receives a signal at a predetermined carrier frequency, including a reference oscillator, which generates a clock frequency having a clock frequency error relative to a specified frequency thereof, a processor, which estimates the clock frequency error and parses the error to determine coarse and fine error correction components, at least one frequency synthesizer, which responsive to the clock frequency and to the coarse error correction component, generates a partially corrected frequency having a residual frequency error, which partially corrected frequency is applied to process the signal, and signal processing circuitry, which applies the fine error correction component to process the signal so as to correct the residual frequency error.
Preferably, the reference oscillator includes a crystal oscillator which is not a temperature-compensated crystal oscillator.
Preferably, the reference oscillator does not receive a control input for adjustment of the clock frequency.
Preferably, the processor estimates the clock frequency error responsive to a known operating characteristic of the reference oscillator.
Preferably, the device includes a sensor, which makes a measurement of an operating condition of the reference oscillator to which the known operating characteristic is responsive, which measurement is used by the processor in estimating the clock frequency error.
Preferably, the sensor includes a temperature sensor.
Preferably, the known operating characteristic includes a variation of the clock frequency with age of the oscillator.
Preferably, the device includes a memory, which stores data indicative of the response of the clock frequency error to the operating characteristic, which data are used by the processor in estimating the clock frequency error.
Preferably, the processor measures the response of the clock frequency error to the operating characteristic and processes the measured response to generate the data stored in the memory.
Preferably, the at least one frequency synthesizer includes first and second frequency synthesizers, and the processor parses the coarse error correction component into a first correction component applied by the first synthesizer and a second correction component applied by the second synthesizer.
Preferably, the device includes a mixer, which mixes the signal with the first partially corrected frequency to generate a first intermediate-frequency signal, which is mixed with the second partially corrected frequency to generate a second intermediate-frequency signal for input to the signal processing circuitry.
Preferably, the device includes a signal detector, which provides, responsive to the signal, an indication of a frequency error remaining after application of one or more of the error correction components.
Preferably, the signal detector includes a frequency estimator, which counts transitions of the processed signal to determine a frequency estimate thereof.
Preferably, the signal detector includes a phase detector, which detects a phase shift in the signal processed by the signal processing circuitry.
Preferably, the signal processing circuitry comprises baseband circuitry.
Preferably, the baseband circuitry includes a subtractor, which subtracts a time-varying phase value from the signal so as to correct the residual frequency error.
Alternatively, the signal processing circuitry includes a complex multiplier, which applies a time-varying phase shift to the signal so as to correct the residual frequency error.
Preferably, the signal processing circuitry compensates for the residual frequency error in the course of processing information carried by the signal.
Preferably, the signal processing circuitry includes a modulator the settings of which are adjusted by an offset determined responsive to the residual frequency error.
Preferably, the signal processing circuitry includes a demodulator the settings of which are adjusted by an offset determined responsive to the residual frequency error.
Preferably, the at least one synthesizer includes a receiver synthesizer and a transmitter synthesizer generating respective partially corrected frequencies, which are respectively applied to process received and transmitted signals.
There is further provided in accordance with a preferred embodiment of the present invention, a radio communication device, which transmits or receives a signal at a predetermined carrier frequency, including a crystal frequency source, an integrated circuit device including a reference oscillator circuit, which is coupled to the crystal frequency source so as to generate a clock frequency and baseband processing circuitry, which processes the signal, and radio frequency processing circuitry, which responsive to the clock frequency processes the signal cooperatively with the baseband processing circuitry.
Preferably, the radio frequency processing circuitry includes at least one frequency synthesizer, which generates a radio frequency responsive to the clock frequency for use in processing the signal.
Preferably, the clock frequency has a clock frequency error relative to a specified frequency thereof, which is corrected cooperatively by the baseband processing circuitry and the radio frequency processing circuitry.
Preferably, the device does not include a temperature-compensated crystal oscillator.
Preferably, the reference oscillator does not receive a control input for adjustment of the clock frequency.
There is further provided in accordance with a preferred embodiment of the present invention, a method for tuning the frequency of a radio communications device so as to correct for a frequency error of a clock frequency generated by a reference oscillator relative to a specified frequency thereof, including estimating the clock frequency error, parsing the estimated error into a coarse error component and a fine error component, responsive to the coarse error component, generating a partially-corrected frequency having a residual frequency error, applying the partially-corrected frequency to process the signal, and responsive to the fine error component, applying a fine frequency correction so as to correct the residual frequency error.
Preferably, estimating the error includes estimating a frequency deviation responsive to an operating characteristic of the reference oscillator.
Preferably, the method includes making a measurement of an operating condition of the oscillator to which the operating characteristic is responsive, and the error is estimated responsive to the measurement.
Preferably, making the measurement includes measuring a temperature.
Preferably, estimating the error includes estimating a variation of the clock frequency with age of the oscillator.
Preferably, estimating the frequency deviation includes recording and using a record of frequency dependence on the operating characteristic.
Preferably, parsing the error into the coarse component includes parsing the error into initial and intermediate error components, and generating the partially corrected frequency includes generating a first partially corrected frequency responsive to the initial error component and a second partially corrected frequency responsive to the intermediate error component, which first and second partially corrected frequencies are applied in processing the signal.
Preferably, applying the partially corrected frequency includes mixing the signal with the first partially corrected frequency to generate a first intermediate-frequency signal, which is mixed with the second partially corrected frequency to generate a second intermediate-frequency signal.
Preferably, estimating the error includes determining, responsive to the signal, an estimate of a frequency error remaining after processing the signal.
Preferably, determining the estimate includes measuring a frequency of a processed signal after application of the partially-corrected frequency thereto.
Preferably, measuring the frequency includes counting transitions in the signal.
Preferably, estimating the error includes detecting a frequency deviation in baseband processing of the signal.
Preferably, detecting the frequency deviation includes acquiring a synchronization of a signal received by the device and detecting a deviation responsive to the synchronization.
Preferably, estimating and parsing the error include estimating and parsing iteratively and applying the iteratively estimated and parsed error components so as to compensate for the clock frequency error to within a predetermined tolerance.
Preferably, determining the estimate includes estimating a frequency error in processing a received signal, and parsing the error includes parsing the estimated frequency error for application of corrections to a transmitted signal.
Preferably, applying the fine frequency correction includes applying the correction during baseband processing.
Preferably, applying the fine frequency correction includes subtracting the frequency correction.
Alternatively or additionally, applying the fine frequency correction includes applying a time-varying phase shift to the signal.
Preferably, applying the phase shift includes driving a complex multiplier which operates on the signal.
Preferably, applying the fine frequency correction includes compensating for the residual frequency error in the course of processing information carried by the signal.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: